The long-term goal of the proposed research is to understand the basis for viral fusion protein-induced membrane fusion with atomic-resolution detail. Fusion of viral and target cell membranes is a key step in infection for many viruses important in disease and a detailed understanding of fusion mechanisms will aid development of anti-viral therapeutics whose mode of action is fusion inhibition. Fusion proteins are also a target of viral vaccine development. The proposed research focuses on the gp41 and the hemagglutinin HA2 fusion proteins of the respective human immunodeficiency virus and influenza virus. These proteins are chosen because the viruses cause major human diseases and because the proteins serve as prototypes for other class I viral fusion proteins. There is particular emphasis on the N-terminal fusion peptide (FP) domains of the fusion proteins, which represent ~20-residue apolar regions at the N-termini of the proteins. Numerous biochemical and biophysical studies have shown that the FP plays a key role in fusion and that chemically synthesized peptides with the FP sequence can serve as useful model systems to understand some aspects of viral/target cell fusion. The long-term objectives will be achieved with three specific aims. Aim 1 focuses on production of multi-mg quantities of large constructs of gp41 and HA2 proteins that contain FPs, including full-length HA2. In addition, gp41 constructs will be made which contain the C-terminal membrane-proximal external region (MPER) and viral transmembrane (TM) domains. MPER and TM are important in fusion and MPER is also an HIV vaccine target. Residue-specific conformation of the membrane-associated constructs will be determined with solid- state nuclear magnetic resonance (SSNMR) measurements of specific carbonyl (13CO) chemical shifts. There will be particular focus on: (1) testing whether structural models of FP and MPER from short peptide studies are valid for these regions in the large constructs; (2) correlating FP and MPER structure to the observed fusogenicity of the construct; and (3) examining whether the gp41 FP is close to either the MPER or TM. Occlusion of MPER by FP could reduce MPER efficacy as an immunogen. Aim 2 focuses on determining the membrane locations of FP, MPER, and TM regions in these large gp41 and HA2 constructs using SSNMR measurements of distances between protein 13CO and lipid or cholesterol 31P, 19F, or 2H. These experiments will test the general structure-function model that deep but not transmembrane FP membrane insertion correlates with greater membrane perturbation and more rapid fusion catalysis. Aim 3 focuses on determining the distribution of antiparallel registries of b sheet FP in several gp41 constructs with very different FP membrane locations and fusogenicities. The free energies of FP membrane insertion will be estimated for the observed registries and correlated with membrane locations and fusogenicities. The overall goal is to connect high-resolution FP structure, membrane insertion depth, and fusogenicity.